Studies with numerous domestic cat embryo lines demonstrated no absolute restriction of RD-114-like virus replication. However, embryos could be characterized as "restrictive" or "permissive" based on whether they were transformed by the RD-114 pseudotype of Moloney MSV. The kinetics of productive infectin of both cell types was one hit. Clones and subclones of "restrictive" cells behaved quantitatively like their parental populations, and showed normal diploid karyotypes. S+L- cell lines, generated from both "restrictive" and "permissive" embryonic cat cells were exploited to study the nature of intracellular RD-114-like virus restriction. These new S+L- cell lines were useful as assay lines for other type C viruses including murine xenotropic and MCF isolates and certain primate viruses. Several FeLV virus-producing cat cells also produced RD-114 like sequences. The sequences were heterogeneous; comprised of those common to all FeLV preparations and those produced only by transformed cells. An RD-114-like virus with a novel coat was further characterized. Preliminary restriction enzyme mapping of its proviral DNA showed no differences from RD-114 DNA other than the 3' "env" region which was 500 bp longer and contained two novel enzyme sites.